This invention relates to silica fillers useful in reinforcing silicone elastomeric compositions. More particularly, this invention relates to compositions for treating silica fillers such that the treated filler improves the mechanical and handling properties of silicone elastomers made with the treated filler.
Silicone elastomers have been widely valued for their resistance to moisture and their high and low temperature stability. Silicone elastomers have been developed which also exhibit excellent handling characteristics. These characteristics are accomplished through the use of treated fillers, usually treated finely divided silica, or by modifiying the curable siloxane polymers.
Treating silica fillers with silicone treating agents is known in the art. For example, U.S. Pat. No. 4,529,774 (Evans et al.) discloses a method for treating silica fillers whereby silica fillers are contacted at a temperature of from 240-310.degree. C. for 4-16 hours while purging volatiles and water and maintaining pressure of from 0 to about 15 psig, with a fluoroalkyl-functional diorganopolysiloxane treating agent, such as a hydrolyzate containing fluoroalkyl-functional cyclic polysiloxanes and low molecular weight diorganopolysiuxoanes. Fluorosilicone gums made using the treated fillers exhibit improved mechanical properties, especially tear strength and compression set, and improved ease of handling in mixing, milling and extrusion.
Another method for treating silica fillers with silicone treating agents is disclosed in U.S. Pat. No. 4,724,167 (Evans et al.). Therein finely divided silica reinforcing filler is contacted at a temperature of at most about 210.degree. C. for a sufficient time to complete the reaction with condensable diorganopolysiloxane which is in the liquid phase and which has hydroxy or alkoxy functionality readily reactive with the silica surface below about 210.degree. C. This method has improved reproducibility and allows for the use of lower reaction temperatures.
The properties of heat cured silicone rubber are largely determined by filler-to-polymer and polymer-to-polymer reactions or interactions. Currently, agents used for the in situ treatment of fillers are intended to facilitate crosslinking between filler and polymer or between polymer and polymer. These materials are primarily silanol-terminated fluids or disilazane fluids which tend to react with silanol end groups on the surface of fillers to reduce the fillers' potential for structuring an uncured compound on standing. These agents may affix fiber-like appendages to the filler either through silanol condensation or through substitution. They may also bridge over other remaining unreacted silanols to preclude them from possible future interaction. Examples of currently used treating agents include fluorosilicone telomeric disiloxanol fluids having an average silanol content of more than 6.0%; polydimethylsiloxane telomeric fluid with an average silanol content of 9 to 12%; 1,3-divinyltetramethyldisilazane; and hexamethyidisilazane.
Occasionally, fluorosilicone heat cured rubber compositions which use in-situ filler treatment are treated simultaneously with several agents such that, for example, one or more agents promote filler-filler crosslinking while one or more other agents simultaneously promote polymer-filler and polymer-polymer crosslinking. Typically, in these compositions, treatment is carried out with fluorosilicone telomeric disiloxanol fluid having an average silanol content of more than 6% and with divinyltetramethyldisilazane (VTDS). The silanol fluid functions as a processing aid as described above, i.e., it reacts with the filler's silanol groups to reduce the filler's potential for structuring. The VTDS acts as a crosslinking agent in that it causes the formation of vinyl moieties on the surface of the filler, which involves the filler in a tighter cure matrix and consequently enhances the durometer, hardness, tensile, modulus, and compression set resistance properties of the heat- or room temperature-cured rubber composition. Typically, only a small amount of VTDS is used to treat the filler.
However, with the treated fillers described above, enhancement of the mechanical properties is accompanied by losses in % elongation and tear resistance properties. This is believed to be a result of the amount of filler used. Good mechanical properties are dependent to a large extent on the crosslink density of the elastomer. Crosslink density, in turn, is a function of the amount of filler present, i.e., the more filler present, the greater the crosslink density. However, the use of filler at amounts which provide good mechanical properties can also result in a loss of elongation and tear resistance.
It would be desirable, therefore, to reduce the amount of filler needed to obtain good mechanical properties so that reductions in elongation and tear resistance properties resulting from excessive amounts of filler can be avoided.
As mentioned hereinabove, the in situ treatment of fillers in silicone rubber compositions usually involves the use of two compounds, one for reacting with the silanol groups on the filler (i.e., the silanol fluid) and the other for promoting crosslinking between filler and polymer (i.e., VTDS). The use of two compounds to treat the filler and promote crosslinking can be time consuming and costly.
Thus, it is further desirable to provide a single compound which functions both as a processing aid and as a crosslinking agent in silicone elastomer compositions.